We have been exploring methods of obtaining biological activity for cDNA- encoded synthetic genomic RNA (vRNA) of human respiratory syncytial virus (RSV). The ultimate goal is to produce infectious virus from a complete synthetic vRNA. This capability would provide an experimental system for analyzing the functions of RSV nucleic acid signals and proteins, and also would be invaluable for the production and characterization of engineered attenuated vaccine viruses. We have constructed cDNAs encoding vRNA analogs in which all of the viral genes have been deleted and replaced with the bacterial chloramphenicol acetyl transferase (CAT) marker gene. When transfected into tissue culture cells and complemented by infection with RSV, these truncated, negative-sense vRNA analogs were "rescued" in that they were expressed, amplified and packaged into infectious particles. Substantial progress has been made in identifying and characterizing cis- acting nucleotide signals important in rescue.